Stoichiometric Conversion ofall trans-β-Carotene to Retinal by Pig Intestinal Extract

β-Carotene and other provitamin A carotenoids are major dietary sources of vitamin A for humans and for many animals throughout the world. Two pathways for this oxidative reaction in mammals are: (1) central cleavage by β-carotene 15,15′-dioxygenase (EC 1.13.11.21) to yield two molecules of retinal per molecule of β-carotene consumed and (2) eccentric (or random) cleavage via several β-apo carotenals to yield one molecule of retinal and various smaller fragments per molecule of β-carotene oxidized. By the use of improved methods to minimize nonenzymatic reactions and to measure isomers of retinal, the mean experimentally measured molar ratio of retinal formed to β-carotene consumed by pig intestinal preparations (800 and 10,000gsupernatants) was 1.88 ± 0.08, close to the theoretical value of 2.0 for central cleavage. β-Apo carotenals, retinol, and retinoic acid were detected, if at all, in trace amounts during incubation. Thus, preparations of pig intestinal mucosa, which is considered a good physiologic model for human intestine, clearly convert β-carotene to retinal, in large part if not solely, by central cleavage. Whether eccentric cleavage plays a greater rolein vivothan it doesin vitrois still uncertain.